When I first heard that there were going to be court appeals against Prop 8, my first reaction was that was really going to be a stretch. It's perfectly appropriate for the Court to find a statute to be unconstitutional, but it's rather problematic to ask the Court to find a constitutional amendment (which is, by definition, now part of the Constitution) to be unconstitutional. However, having read the briefs that have been filed, I see there actually is a claim to be made, and I'm cautiously optimistic that the court may go for it. (Of course, I also cringe now just saying "cautiously optimistic", as I used that same phrase often in expressing my hope that Prop 8 was going to fail at the ballot box. I guess with me, cautious optimism springs eternal.) The gist is that the California Constitution has two different processes for being changed, one called an "amendment" and one called a "revision". An "amendment" (which is how Prop 8 was filed) simply requires a bare majority of the voters, while a "revision" requires a more cumbersome process, first a two-thirds majority vote in both legislative houses and then going to the voters. The notion is that items of limited scope are "amendments", while those that make more substantive changes in the constitution of the government must be "revisions". The Constitution itself provides no more clarity on exactly what makes something rise to the level of a "revision". That distinction is left up to the Court to interpret. And historically, the Court has invalidated a couple of previous ballot initiative amendments, saying that they were too fundamental to be an amendment and really needed to go through the revision process. That's what Prop 8 opponents are asking the Court to do with Prop 8.
There's a good argument to be made that a change to the Constitution that actually repeals fundamental rights is substantive enough in scope to warrant the revision process, and the three appeals that I've read (I've heard there may now be a fourth) have made that argument. This would actually vindicate the intuition of many people that it's outrageous that the revoking of a fundamental right could be put to a ballot initiative for a bare majority. For those who care about constitutional forms of government, Prop 8 sets a frightening precedent (and it is indeed unprecedented). If that precedent is allowed to stand, then any unpopular minority group could have its constitutional rights revoked by a bare majority of the voters. Conceivably, we could put up an initiative that Mormon marriages would no longer be recognized by the state, or even more egregiously, California could legalize slavery again. Is it reasonable to think that California voters could enact such changes with just 51% of the voters on a ballot initiative? Undoubtedly, such measures would be struck down on appeals to the US Constitution, but one would hope that the California Constitution's guarantee of fundamental rights would stand on its own in this regard. Our constitutional government provides for "checks and balances" in separate branches of government, and one of the important duties of the Supreme Court is to safeguard the constitutional rights of minorities against what the founding fathers termed the "tyranny of the majority". A measure like Prop 8 defies that fundamental separation of powers. That's the gist of the arguments being made, and the Court may well find them compelling. Governor Schwarzenegger has advocated as much, and 44 California Senators and Assemblymembers have signed on to an amicus brief in favor of that position.
Of course any Supreme Court decision ultimately comes down to the seven Justices making the decision. The decision last spring recognizing a fundamental right to equal recognition of same-sex marriages was made by the minimum of four Justices, with three dissenting. For the original four concurring Justices, if they really believe what they wrote last May (their language was pretty clear and strong in terms of fundamental rights and protecting a minority group), then they would stand up for their opinion, and strike down Prop 8 as an insufficiently enacted constitutional revision. Those Justices were courageous in the face of what they probably knew would be substantial popular backlash (protecting minority rights, as the Court has noted, is often an unpopular job), and we can hope their courage does not falter. Unfortunately, ominous rumblings about their re-election prospects, and calls to remember Rose Bird, are being made by the Prop 8 backers (who have proven how ruthless they can be). Then we have the three dissenting Justices. The underlying issue for them will be whether they take the earlier decision as established precedent when evaluating this claim, or whether that precedent (that they didn’t agree with) is still open in their minds. Technically, this is a new and purely procedural issue (i.e., whether the initiative was inappropriately put forth as an amendment) that doesn't invite a revisiting of the earlier decision (whether there is a fundamental right to marry the person of one's choice), but in practice it may be hard to resist the opportunity to implicitly revisit the earlier decision in deciding this one. On the other hand, if I recall correctly, when the Prop 8 proponents requested a stay of the final Marriage decision pending the election, I believe that was unanimously denied by the Court, indicating that the dissenters are swayed by at least some arguments for proper procedure and not revisiting settled matters. Judging by the opinions filed in the Marriage decision (in addition to the decision, there was one concurrence and two separate dissents), I'd say the most likely outcome is that Prop 8 is invalidated by the same 4-3 decision as the original Marriage decision, since the four concurring justices spoke strongly about the role of the Court in protecting rights of minorities, while the three dissenters spoke strongly about the Court not overturning the will of the people expressed at the ballot box, and those same issues arise here.
The filings can all be read here (update: I just heard there are now six separate filings). Of those I've read, the NCRL brief does the most technical work in discussing the various precedents on the revision vs. amendment issue, but the brief by the Cities makes the most readable and very eloquent statement of the issue. If you read only one, read that one.
There are rumblings that the Court will take some action on Wednesday, making initial decisions whether to even hear the case (I think they will, since both sides are urging that), and whether to issue an immediate stay in the meantime (I suspect they won't). Stay tuned.